Hi David,
Based on the descriptions I'm seeing, such as "
Hafler [22] introduced a 100pF capacitor from driver V4 anode to R4 to achieve additional 12dB margin, with bandwidth reduced from 200kHz to 80kHz", I'm thinking that the 'direction' of the feedback is from the plate of the diff amp back to the cathode of the input tube. If I read your analysis correctly, you are positing that the feedback arrangement is from the secondary of the OPT to one plate of the diff amp. (a'la Schade, sort of...).
In my scenario there are two 'sources' (OPT & Driver tube plate) and one 'sink' (Input tube cathode).
In your scenario, there is one 'source' (OPT) and two 'sinks' (Driver tube plate & Input tube cathode).
I can't tell you which one is correct......
I'm not that good.....
Based on the 80kHz pole that is mentioned in the historical text, I believe we are looking for a resistance in the original circuit of approx. 20k that is paired with the 100pF to give a corner frequency of ~80kHz.
Sorry to have muddied the waters, perhaps....
Bruce
David McGown wrote:
The Dynaco circuit has a 100pF capacitor from the feedback loop to the plate of the inverted side of the driver tube that is there for high frequency compensation to limit response above 80 kHz and provide 12dB of stability margin. I believe this capacitor acts into the impedance at the plate of the driver, being the parallel resistance of anode resistance and plate resistor, essentially forming an CR network high-pass filter. Therefore, using an ECC99 will end up changing this, since the impedance is different. Based on a nifty online filter calculator for a CR high pass circuit, and modeling the original circuit values, the 100pF capacitor is acting into the parallel resistance of 9500 ohms (6SN7 at the op-point) and 47K plate resistor, or 7900 ohms. This results in a pole at around 200kHz. With an ECC99 with an anode resistance of 2300 ohms and plate resistor of 20k results in a parallel resistance of 2060 ohms. The feedback capacitor would need to increase to 390pF to maintain in the same 200kHz pole. Does this sound right to you? (Anyone else please chime in)
David